The case for a monopoly has been put - and it's one I agree with - but I' with camperbob here, patent monopolies are too long. They don't respect the change in product lifecycles that's come about in the last century or so.

If it took 10 years to produce the next line of products and if products were manufactured to last that long then 15-20 years seems fine.

But the patent term is far too long compared to the product lifecycle (in computing/electronics fields at least); patented tech is too often redundant by the time it enters the public domain.

I'd like to see maybe 5-8 year terms and possibly with a cap that looks something like "if global revenue is 100 times the number of inventors times 50 years times the 90th percentile pay in the country where patented the patent lapses" (the idea being selling at a 1% patent premium that returns enough to reward the inventors with never having to work again means the patent should lapse) [but admit that would be a very hard law to draft without leaving loopholes; there's probably major flaws too, one really would rather look at profit but it's too easily manipulated I feel].

Anyway, can someone justify the 15-20 years over a 5-8 year term (for electronics, say)?

I don't disagree with any of this. What's a sensible patent term in aerospace, where a single design might take a decade to bring to market, and what's sensible in consumer electronics isn't the same.